In recent years, practical use of hybrid automobiles and electric automobiles is increasing, and regarding driving motors and motors for generators used in these automobiles, strong demands are being made for higher efficiency and higher output.
Further, the development of driving systems for motors has made frequency control of the driving power source possible, and motors for variable speed operation or high speed rotation exceeding commercial frequency are increasing.
Therefore, strong demands are being made for higher efficiency and higher output, i.e. lower iron loss and higher magnetic flux density for non-oriented electrical steel sheets for iron cores used in motors such as above as well.
In order to reduce iron loss of non-oriented electrical steel sheets, a means of reducing eddy current loss by increasing the contents of for example, Si, Al, and Mn, etc. and increasing electric resistance has been generally used. However, with this means, there was a problem in that a decrease of magnetic flux density cannot be avoided.
Under the situation, some proposals for methods for improving the magnetic flux density of non-oriented electrical steel sheets have been made.
For example, JPH680169B (PTL 1) proposes a method for obtaining a higher magnetic flux density by setting the P content to 0.05% to 0.20% and Mn content to 0.20% or less. However, when these methods were applied to factory production, there were problems such as the fact that troubles including sheet breakage were likely to occur during the rolling process, etc., and reduction in yield or line stop was unavoidable. Further, since the Si content is a low amount of 0.1% to 1.0%, iron loss was high, and iron loss properties in a high frequency were particularly poor.
Further, JP4126479B (PTL 2) proposes a method of obtaining a higher magnetic flux density by setting the Al content to 0.017% or less. However, with this method, sufficient improving effect of magnetic flux density could not be obtained from a single cold rolling at room temperature. Regarding this point, by performing cold rolling as warm rolling with a sheet temperature of around 200° C., although magnetic flux density will improve, there was a problem in that adaptation of equipment for warm rolling or process management due to restriction of production would be necessary. Further, cold rolling of twice or more with intermediate annealing performed therebetween would increase manufacturing costs.
Further, as elements other than the above elements, the addition of Sb and Sn are known to be effective for obtaining higher magnetic flux density, and for example, JP2500033B (PTL 3) discloses such effect.
On the other hand, as a manufacturing method, JP3870893B (PTL 4) discloses a technique for performing hot band annealing as box annealing on a material with P content of more than 0.07% and 0.20% or less, and setting the grain diameter before cold rolling to a particular range. However, with this method, it is necessary to set the soaking temperature of hot band annealing to a fixed range in order to set the grain diameter before cold rolling to a certain range. Therefore, if continuous annealing which is excellent in productivity is applied, in particular, when the preceding or succeeding steel is a different type from the steel in question, there was a problem in that variation in properties increases. Further, PTL4 discloses that better magnetic properties can be obtained by performing hot band annealing at a low temperature for a long period and setting a low cooling rate.
As mentioned above, with conventional techniques, it is difficult to stably provide non-oriented electrical steel sheets having high magnetic flux density and excellent productivity (manufacturability) using material with sufficiently low eddy current loss and Si content exceeding 3.0%, at a low cost.